A hydrofoil watercraft typically consists of a displacement hull boat to which is attached "wings" or hydrofoils that generate lift as they travel through the water, much in the same way that the airfoil design of aircraft wings provide lift in the air. When the hydrofoil craft is operating at low speeds in what is termed the "hullborne mode," the hull functions as a conventional displacement hull to support the craft on the water. As the craft attains higher speeds, the lift provided by the flow of water over the hydrofoil is sufficient to lift the hull entirely clear of the water. At this point, the craft is operating in the "foilborne mode." Once out of the water, the hull no longer suffers resistance from friction with the water, or from waves in rough water, so that higher speeds and a more stable ride can be attained.
Propulsion systems for commercial hydrofoil craft usually consist of marine diesel engines which drive propellers at the end of long inclined shafts that project from under the hull. The use of water propellers in the foilborne mode limits the top speed of the craft because water propellers become inefficient at the higher speeds at which hydrofoil craft are capable of operating and require greater horsepower. One method for overcoming this drawback is to use water jets. Although this method has provided some increased speed over water propellers, the intake openings and additional equipment of the water jets increases weight and creates drag that will limit the speed of the craft.
One proposal for overcoming these disadvantages is to use a propulsion system that does not rely upon the water, such as air propellers or jet engines. The drawback to this proposal is that maneuverability becomes very difficult when the hydrofoil craft is operating in the hullborne mode. In addition, the air propellers are not as effective as water propellers in accelerating the craft to the transition speed. Hence, there is a need for a propulsion system for hydrofoil craft that provides acceleration and maneuverability when the craft is in the hullborne mode and also provides efficient maximum thrust for high speed operation in the foilborne mode.